The present invention generally relates to a method and an apparatus for controlling a moving web. More specifically, the present invention relates to a web guide apparatus having minimal mechanical backlash cooperating with a high speed control system which allows for precise control of a transverse location of the moving web. The present invention further includes a method of controlling the transverse location of the web.
Generally, there are two types of guide systems for controlling a transverse position of a moving web. A first type of guide system for controlling a transverse position of a moving web is a passive system.
An example of a passive system is a crowned roller, also called a convex roller, having a greater radius in the center than at the edges. Crowned rollers are effective at controlling webs that are relatively thick in relation to the width of the web such as sanding belts and conveyor belts.
Another passive type of guide system is a tapered roller with a flange. The taper on the roller directs the web towards the flange. The web edge contacts the flange and thereby controls the transverse position of the web. A tapered roller with a flange is commonly used to control the lateral position of a narrow web, such as a videotape.
However, a passive guide system cannot guide wide, thin webs because, depending on the type of passive guide system, either the edge of the web tends to buckle or the web tends to develop wrinkles. To effectively control a wide, thin web an active guide system is required.
A typical active guide system includes a sensing device for locating the position of the web, a mechanical positioning device, a control system for determining an error from a desired transverse location and an actuator that receives a signal from the control system and manipulates the mechanical positioning device. A typical control system used for actively guiding a thin, wide web is a closed loop feedback control system.
Typically, a web to be processed has been previously wound onto a spool. During the winding process, the web is not perfectly wound and typically has transverse positioning errors in the form of a zigzag or a weave. When the web is unwound, the zigzag or weave errors recur causing transverse web positioning problems.
In precision web applications such as webs used in optics and electronics, the transverse location of the web must be precisely controlled. Most commercially available active web guide systems are not capable of controlling the transverse location to the level of precision required for these web applications. Commercial web guides typically employ rod ends, belts, sheaves, slides and threaded nuts and bolts, each of which has some mechanical play. Often, in a commercially available guide, the total mechanical play is in range of 125-375 microns (0.005-0.015 inches). A control system cannot guide a web to within a range of the guide's backlash or mechanical play.
While the control system of a commercially available web guide has some error, often the error caused by the control system is insignificant when compared to the error caused by the mechanical backlash or play in the guide. The mechanical backlash, without accounting for any other error can preclude many commercially available web guides from being used for precisely locating a transverse location of a moving web.